There are two points that can be adjusted on the Model 116 dial drive in the event that there is any slipping or roughness [Assembly Part No. 31-1563 – Also used on models 116, 640, 641, 650, 660, and 680].
First remove the lock spring “A” from the assembly and then, by pulling on the Speed Shaft “B”, both the speed shaft and the vernier shaft, as well as the bearings and the tension spring “E”, can all be removed from the housing. Through the use of two blades, placed at each end of spring “E” and expanding with pressure, this spring can be stretched. Normally the spring measures 9/16 inch, and after stretching 1/8 inch, this will be sufficient to increase pressure on vernier shaft “F” when re-assembled. Part number of inside tension spring is 28-8366.
The second adjustment is effected when tightening the upper bracket of the drive in place (not illustrated). When enmeshing the semi-circular ring with the drive mechanism, place the bracket in such a position as to allow the ring to fit well into the spring slot, but not all the way. This allows for any irregularity in the ring itself and completes the necessary adjustment.
From the Philco Serviceman – September, 1935.